Append python script for Blender 2.8
With some help on twitter, this code is working now.

import bpy

bpy.ops.wm.append(
    filepath="cube.blend",
    directory="/home/lucas/Desktop/cube.blend\\Object\\",
    filename="Cube")


Basically,

filepath= .blend file name

directory= directory of your file and the last part is the folder inside the blend file.

filename= Is the object, material, collection or whatever you want to append.